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This chapter is concerned mainly with bacterial infections of the central nervous system (CNS), with manifestations including bacterial meningitis, septic thrombophlebitis, brain abscess, epidural abscess, and subdural empyema. The granulomatous infections of the CNS, notably tuberculosis, syphilis and other spirochetal infections, and certain fungal infections are also discussed. In addition, consideration is given to infections and infestations caused by rickettsias, protozoans, worms, and tick-borne infections.
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A number of other important infectious diseases of the nervous system are discussed elsewhere in this book. Viral infections, because of their frequency and importance, are allotted a chapter of their own (see the following Chap. 32). Diseases caused by bacterial exotoxins—diphtheria, tetanus, botulism—are considered with other toxins that affect the nervous system (see Chap. 41). Leprosy, which is essentially a disease of the peripheral nerves, is described in Chap. 43, and trichinosis, mainly a disease of muscle, in Chap. 45.
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BACTERIAL INFECTIONS OF THE CENTRAL NERVOUS SYSTEM
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Bacterial infections reach the intracranial structures by one of two pathways, either by hematogenous spread or by extension from cranial structures adjacent to the brain (ears, paranasal sinuses, osteomyelitic foci in the skull, penetrating cranial or congenital sinus tracts) (see Durand et al; Thigpen et al). Some infections are iatrogenic, being introduced in the course of cerebral or spinal surgery, the placement of a ventriculoperitoneal shunt, or, rarely, by a lumbar puncture needle. Increasingly, craniospinal infections are nosocomial, that is, acquired in-hospital; in urban hospitals, nosocomial meningitis is now as frequent as the non–hospital-acquired variety (van der Beek and colleagues).
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The mechanisms of hematogenous spread remain an area of investigation. In most instances of bacteremia or septicemia, the nervous system is not infected; yet sometimes, a bacteremia caused by pneumonia or endocarditis is the only apparent predecessor to meningitis. With respect to the formation of brain abscess, cerebral tissue has a notable resistance to infection. Direct injection of virulent bacteria into the brain of an animal seldom results in abscess formation. In fact, an abscess is consistently produced only by injecting culture medium along with the bacteria or by causing necrosis of the tissue at the time bacteria are inoculated. In humans, infarction of brain tissue because of arterial occlusion (thrombosis or embolism) or venous occlusion (thrombophlebitis) may be a common and perhaps necessary antecedent by way of causing of a necrotic nidus.
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The cranial bones and the dura mater protect the cranial cavity against the ingress of bacteria. In addition, the cranial epidural and subdural spaces are practically never the sites of blood-borne infections. However, these protective mechanisms may fail if suppuration occurs in the middle ear, mastoid cells, or frontal, ethmoid, and sphenoid sinuses. Two pathways from these sources have been demonstrated: first, infected thrombi may form in diploic veins and spread along these vessels into the dural sinuses, and then, in retrograde fashion, along ...